A scanner or a similar type of device typically captures an image of a document while conveying the document, using an imaging device, such as a line sensor where imaging elements are one-dimensionally arrayed. If a dirt substance, such as paper dust, other fine particles, glue, etc., adheres to a glass surface of the imaging device, noise line extending in a document conveyance direction is generated in an image of a document. Thus, a scanner or an image processing apparatus connected to a scanner, such as a personal computer, needs to warn a user when there is dirt at the imaging position.
There has been disclosed a facsimile machine that checks the presence of dirt in a document reading system over a plurality of steps using a determination module that compares white reference data read by an image sensor before reading a document with a plurality of white level data set in advance. The facsimile machine records a message on a recording paper and outputs it according to the status of dirt when the white reference data read by the image sensor exceeds each white level data (refer to Japanese Unexamined Patent Publication (Kokai) No. 05-292317).